JP-A-9-54566 discloses a display with EL elements having a phosphor made of ZnS:Mn. However, such El elements only emit an umber color light so that the EL device cannot be used as a display. Accordingly, other displays with EL elements emitting other colors are now being developed.
As for printer technology, a printer head having LEDs as a light source is used for an LED printer. However, because light emitted from respective LEDs is imbalanced, the printing quality of the printer is not good. Accordingly, JP-A-5-221019 discloses a printer head having EL elements having a phosphor made of ZnS:Mn.
Regarding the EL elements having the phosphor made of ZnS:Mn, an emission decay time of the phosphor is several seconds longer than an emission rise time thereof because the emission rise time is several microseconds as shown in FIG. 25. As a result, print dots extend in a direction parallel to a paper transmission direction when printing speed increases.
FIG. 26A illustrates an exemplary circular dot and an elliptical dot extended due to high printing speed, and FIG. 26B illustrates dots when a plurality of circular dots and a plurality of elliptical dots are printed at intervals of one dot. As shown in FIG. 26B, the elliptical dots overlap one another when respective dots are printed with intervals corresponding to one dot. To avoid an overlap of the elliptical dots, the printing speed or resolution of the printer is consequently decreased.
To solve the problem mentioned above, a printer head with EL elements having a phosphor of which an emission decay time is shorter than that of a phosphor made of ZnS:Mn is used for a printer light source. However, the luminous power of the phosphor is not sufficient. When EL elements having such a phosphor are used for a display, the luminous power of the phosphor is also not sufficient. This is because people perceive light based on an amount of light integrated over time, and the amount of light integrated over time decreases due to a short emission decay time.